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hphien
16-10-2005, 06:08 AM
Hi all,
Could anybody help me?
Can I install refrigeration system for cold storage 1000MT(room temp. -25oC), refrigerant NH3, liquid suplly thermostaic expansion valve.

Thanks and best regard,
Hphien

Peter_1
16-10-2005, 09:29 AM
Sure, I know that Danfoss has these valves in their program and certainly other manufactures

US Iceman
16-10-2005, 10:38 PM
Evaporator manufacturers usually mention a lower limit on the use of TXV's for ammonia. The -12C temperature is approaching the lower limit we use here in the US.

At low temperatures (with ammonia) the usual feed method is flooded or liquid overfeed.

This can be especially helpful if the system experiences capacity changes.

You should also be aware that the ammonia TXV's will have to be rebuilt or replaced on a regular basis. The pins and seats wear out in about 2-5 years. They are also dependent on liquid subcooling to last this long.

For big systems, liquid overfeed would be the preferred method.

hphien
18-10-2005, 04:31 AM
Thanks you very much
In this application, i will use liquid overfeed system for big store.
Can i use DX for small store?

US Iceman
18-10-2005, 03:41 PM
The short answer is yes.

However,....

I have found that if the number of evaporators used in the cold storage space increases, a liquid overfeed system can become cost effective.

When using TXV's for ammonia service you still need a suction accumulator. You also need some liquid subcooling to maintain the liquid quality entering the TXV.

You might also include a liquid transfer system to pump liquid from the suction accumulator to the high pressure receiver, when the accumulator liquid level increases.

BY the time you add up the equipment costs for all of this and the installation costs, liquid overfeed system start to become cost effective on small systems.

I've worked on too many ammonia TXV systems to say I would install one. Just my opinion...

TXiceman
08-11-2005, 12:25 PM
Generally ammonia TXV's are not applied beow about 0 dF. Based on my experience, I try to limit their aplication to now lower than +10 dF.

Ken

lana
11-11-2005, 02:48 PM
If ammonia is used in a DX system wouldn't there be an oil return problem?:confused:

US Iceman
11-11-2005, 09:09 PM
Not really, unless the evaporator piping is trapped. For the most part, DX with NH3 works OK as long as the evaporating temperature is not below about 0F.

I prefer not to use DX on ammonia for other reasons.

Sabroeclaus
11-11-2005, 10:43 PM
Hi. This is proberly not the right place to ask but i try anyway.
I just commisioned a Vilter 1501 in Africa i just want to ask US Iceman.Do you think ther is any way to get the Vission control to show the pressure and temp in BAR/Celcius.as my client dont understand F/psig

I just ask as i know you worked for Vilter before.
When i ask Vilter they dont seem to understand the ;Problem:

Best Regards
Claus

lana
12-11-2005, 06:14 AM
Not really, unless the evaporator piping is trapped. For the most part, DX with NH3 works OK as long as the evaporating temperature is not below about 0F.

I prefer not to use DX on ammonia for other reasons.

Dear Iceman,
Would you please specify the other reasons?
Ammonia is not miscible with its oil and the oil is heavier than the liquid ammonia therefore, in the evaporator the oil stays below the liquid and will trap there. That's why generally liquid overfeed is used and the oil is separated in the separators and receiver. I would appreciate if you could elaborate on this.
Cheers:)

US Iceman
12-11-2005, 05:35 PM
Historically, flooded evaporators were used primarily in ammonia refrigeration systems. One of the major concerns with flooded evaporators is a need to periodically drain oil from the evaporator and surge drum.

In large installations with many evaporators this required a lot of time and personnel. Sometime in the 1920's someone started to use a pump to circulate liquid refrigerant to the evaporators. A common vessel was used as a source for the liquid refrigerant and pumps. The vessel was also used as a liquid/vapor separator.

This became the liquid overfeed system.

With a single vessel, the oil draining process became much easier. It also became more cost effective to install and operate, so many systems installed today use this method of pumping liquid to the evaporators.

Oil separates from liquid ammonia because the specific gravity of the oil is greater than the liquid refrigerant. Oil in an ammonia system will settle out in any low velocity region of the system. This can be piping , heat exchangers, or vessels.

When DX systems are used on ammonia several things seem to happen all of the time. People design systems with no subcooling and the TXV's wear out prematurely. Ammonia TXV's will also wear out if subcooling is provided, but they will last longer. The flash gas either in the liquid line or that created by the expansion process causes wire drawing in the valve pin and seat.

This leads to valve hunting and poor control, so you have flooding problems. Recognizing this, some install liquid transfer systems. Another layer of complexity and cost. You should still have a liquid/vapor separator to protect the compressors.

With DX systems you need a minimum pressure difference to achieve the valve capacity. This means you need discharge pressure controls, which results in higher power input to the system.

The large volume of vapor generated at low temperatures can create problems with DX ammonia systems. This refers to coil circuiting and heat transfer and therefore the coil manufacturers traditionally limit the minimum evaporating temperature to about 0F.

You do not have any of these problems with a properly designed liquid overfeed system.

Evaporator coils for liquid overfeed have about 10-15% more capacity than the same coil size circuited for DX ammonia.

Properly designed, a liquid overfeed system is more cost effective, more energy efficient, and easier to work on.

Some people have the opinion that liquid overfeed systems are too expensive to install, or that DX is much better. When the long term consequences are reviewed, liquid overfeed is a much better alternative.

Sabroeclaus
12-11-2005, 08:47 PM
Hi Us Iceman.

I thank you for the reply.I have solved the problem by asking John Lancaster at Vilter.(You just have to know whom to ask)

You can switch o BAR/C But only in the main picture.All the setup settings is displayed in PSIG/F..??

I hope we can convince Vilter that it has to be an option as we are going to be the Vilter dealer in Africa/Middel East and Europe.I mean that its difficult to get a engnieer in Angola to start to learn PSIG/F if he is used to deal with BAR/C .Anyway nowh i am in Malawi to commision a 16 cyl Vilter.They seems to like this compressores after Sabroe give the news that they stop making the SAB Screws.

Thanks for the help.

lana
13-11-2005, 04:53 AM
Hi Iceman,

Many thanks for your very good explanation.
Cheers.:)

TXiceman
13-11-2005, 08:58 PM
The problems I have seen using ammonia DX is the circuiting and circuit loading or velocity. Since ammonia is generally about 1/3 of the mass flow of the "*****" refrigerants, cooler or coil circiting can be come critical.

With the circuiting we also get the oil return problem. Due to the higer refrigerant mass flows of the "*****s" you will naturally have more opportunity to move the oil which mixes with the refrigerant to varing degres. With ammonia, the oil and refrigerant do not mix and the oil wants to drop out as soon as you get some velocity reduction. The oil pools readily and you get oil not coming back to the compressor or in larger volumes.

The lower temperatures only aggriavate the oil return as the oil gets viscous where an oil that mixes with the refrigerant will be thinner and more likely to be returned.

Oil return in flooded ammonia sytsems is handled with oil pots or oil stills to allow the ol to settle out where it can be heated and returned to the compressor.

Last DX ammonia chiller I was involved with, the heat exchanger vendor finally gave up and converted the DX chiller vessel to a thermsiphon. We were running a 10 dF ET. We monted a thermosiphon drum over the exchanger and changed out the heads for a one pass design. Worked like a champ after the conversion.

Ken

Ken

US Iceman
13-11-2005, 09:03 PM
...converted the DX chiller vessel to a thermsiphon.

This is a great method to use. I have worked on about 10 projects were this was performed. All of them worked much better than a DX system.

TXiceman
23-11-2005, 06:59 PM
US,

Sounds like you and I have hoed the same row of cotton more than once.

Ken

US Iceman
23-11-2005, 07:13 PM
Ken,

I'm pretty sure both of us have plowed over the same fields. :D

Since we both have been in the business for about the same amount of time, I would not be surprised if we both had the same stories to tell.

The names and places may change, but the same problems always seem to occur.

Thanks for the laugh. I had not heard that expression for a long time.

Mark C
04-12-2005, 09:06 PM
US,

Sounds like you and I have hoed the same row of cotton more than once.

Ken

You guys are not the only ones...

To simplify... Superheat is the bane of an ammonia refrigeration system. DX systems demand superheat to function, for which ammonia systems take a large penalty.